One stage of integrated circuit design involves determining where bond pads are to be located on the die or chip. Once functionality and circuit requirements are met in the design, the required number of bond pads to operate the device can be determined. The placement of the required number of bond pads is not a trivial design task. For example, capabilities of manufacturing processes must be taken into account. Moreover, the placement must not unnecessarily increase the final die size. Even in designing devices which are to use wire bonding (as opposed to tape automated bonding or flip chip bonding using conductive bumps), the placement of bond pads can be critical.
Several techniques exist for determining appropriate positions for bond pads in devices which are to be wire bonded around the die periphery. One of the simplest and most prevalent techniques is to simply divide the available perimeter of the semiconductor die by the required number of bond pads. The result is a bond pad configuration having a constant pad pitch (where pitch is the distance from the center of one bond pad to the center of an immediately adjacent pad) around the entire die periphery. A problem with this method is that the constant pad pitch calculated may not be sufficiently large to accommodate bond pad cells. A bond pad cell in a semiconductor design or layout includes not only the actual bond pad metallurgy but other components which designers may include for electrical integrity. These other components may include circuitry to protect from ESD (electrostatic discharge) damage, input buffers, or the like. As a result, the bond pad cell area is usually larger than just the bond pad itself. Accordingly, using the aforementioned constant pitch calculation may result in a bond pad pitch which is smaller than the bond pad cell. In such an instance, the die is said to be "bond pad limited," in that the die size is too small to accommodate all bond pad cells. A solution typically used to avoid the problem of being bond pad limited is to increase the size of the die to accommodate the required number of bond pads and associated cells. However, increasing the die size is an unacceptable solution when the market demands smaller and smaller device sizes to remain competitive.
Another problem in using a constant pad pitch layout is that the "effective pad pitch" or "wire pitch" (the actual pitch between immediately adjacent bonding wires) is not constant around the die periphery, unless all the wires are parallel. Instead, the effective pad pitch (wire pitch) decreases as the die corners are approached because the leads to which the bond pads are eventually bonded are at a larger pitch than the bond pads. This effect is illustrated in FIG. 1. In FIG. 1, P equals the constant bond pad pitch and P' equals the effective pad pitch (wire pitch). As wire bonds 10 approach corners of a semiconductor die 12, an angle .theta. increases due to an increasing pitch of leads 14. As a result, the effective pad pitch or wire pitch (P') is reduced according to the equation: P'.sub.x =P*Cos .theta..sub.x. The effective pad pitch is an important parameter because this distance effects the ability of a wire bonding tool to make bonds without the capillary of the tool disturbing previously made wire bonds. Use of constant pitch bond pad layout may not lead to a layout suitable for manufacturing if the effective pad pitch (wire pitch) becomes too small.
A proposed alternative has been to use a first order approximation of a constant effective pad pitch to determine the location of the bond pads, rather than use a constant pad pitch. This has been proposed using either of two different methods. A first method is termed a "radial layout," while a second method is termed an "exponential layout." Both methods are described in, "I.C. Package Inner Lead and Chip Bond Pad Layout Recommendations for Robust Package Manufacturing," by Wyatt Huddleston et al., 1993 International Electronics Packaging Conference Proceedings, Sep. 12-15, 1994, p. 694-702. In the radial layout, the idea is to define the bond pad locations such that the wire pitch is equal all around the die. This is achieved by assuming that the final wire bonds will be equally arrayed from .theta.=-45.degree. to .theta.=0.degree. back to .theta.=45.degree. along each side of the die. This is usually the case in most high lead count QFP (quad flat pack) packages. In using a constant wire pitch, it follows that the pad pitch is increased from a minimum at the die centerlines to a maximum at the corners since the wire pitch is governed by the equation P'.sub.x =P*Cos .theta..sub.x, where .theta..sub.x increases from 0.degree. at the die centerlines to about 45.degree. at the die corners. Predicting the positions of the pads for this type of layout requires iterative calculations. Primary inputs to these equations are the lead count and desired die size and the outputs are bond pad positions and resulting wire pitch. The advantage of the radial layout is that the effective wire pitch (wire pitch) approaches a constant value. This results in more uniform wire distribution, and hence, a more uniform wire bond process.
In the exponential layout, the lead frame design is also accounted for in determining the bond pad layout. Whereas in the radial concept the angle between the wires and a line perpendicular to the die edge was assumed to range from 0.degree. at the die centerline to 45.degree. at the die corners, the exponential approach does not make this assumption. The wires may range up to a value less than or greater than 45.degree., depending on the actual position of the second bonds (i.e. the bond to the lead frame). In the exponential layout, the pads are positioned to create a constant effective pad pitch (wire pitch) taking into account the actual position of the second bond, whereas in the radial layout the second bond position is ignored. The exponential pitch derives its name from the attribute that the resultant pad pitch asymptotically approaches the pitch of the lead tips and follows an exponential curve (with the exponent&lt;1). For this exponential layout, the bond pad pitch is incrementally increased along the die edge so as to maintain a constant wire pitch, but at a different rate than with the radial layout method. As can be ascertained from the above descriptions, the exponential pitch pad layout relies upon knowing the lead tip layout and conversely the lead tip layout relies on the pad layout. Hence, not only do the bond pad locations result from an iterative calculation, the lead tip positions also result from an iterative process. This method requires a concurrent design methodology as both the bond pad and the inner lead tips are codependent.
While each of the radial and exponential layouts provide advances over constant pad pitch layouts, both have problems. The exponential layout method requires knowledge of many variables: the total bond pad count, the die width, the die length, a corner gap of the die (defined later), a street distance of the die (also defined later), the minimum lead tip width, the minimum space from lead tip to lead tip, the tie bar width, the minimum distance between the tie bar and the cornermost lead, and the distance from the lead tip to the point of the second bond (i.e. the bond to the lead). Knowledge of all these variables may be available if the die is to be packaged in an existing package design, but in many instances a new package design is needed or desired, in which case some of these variables are unknown. The radial layout requires the use of only five variables: the total bond pad count, the die width, the die length, a corner gap of the die (defined later), and a street distance of the die (also defined later). As a consequence of requiring fewer variables, the radial layout has a drawback of potentially determining bond pad coordinates which place the pad centers at distances closer than is physically possible. By not accepting the geometric constraints of the bond pad size and cell size, the radial positions could result in overlapped pads. In general, both the radial and exponential layout techniques fail to account for processing limits imposed by manufacturing equipment and design constraints other than pad pitch. Accordingly, a need for a new layout method exists.